Electrical Grounding Techniques
Various electrical grounding techniques are utilized throughout the world for the prevention of electrical damage to buildings and equipment. Such grounding techniques find numerous applications in such diversified areas as power and telecommunication systems, electronic equipment, fuel storage tanks, industrial installations, commercial and residential buildings as well as buried equipment such as pipelines. Two distinct types of grounding techniques are commonly employed. The first involves the protection of personnel, buildings or equipment from any of a number of rapid and/or intense electrical hazards, such as a lightning strike. The second technique, known as “cathodic protection” employs activated grounding to reduce the slow degradation of metallic installations by electrochemical corrosion.
The established grounding techniques commonly involve the use of wires or rods of copper or other electrically conductive metals being attached to the installation requiring protection, after which the metallic rod or conductor is buried in a narrow trench or driven into the earth. In the cathodic protection technique, an electrical circuit is established such that a direct electrical current flows into buried electrodes. The electrical circuit is designed such that the metallic structures (such as pipes etc.) which require protection become the cathode, while the conductive material which forms the anodes is imbedded in the earth some distance away.
The art of electrical grounding may also be conveniently divided into two classes: “shallow trench” and “deep well” applications, with the latter technology being primarily used for cathodic protection. This shallow trench grounding method is described in greater detail in our co-pending U.S. patent application Ser. No. 11/124,243 filed May 9, 2005, the entire disclosure of which is incorporated herein by reference, as are the disclosures of the prior U.S. patents of others specifically identified in the present specification.
One particularly valuable material used in both shallow and deep well applications consists of various forms of carbon. Carbon is allotropic and is found widely in its crystalline and amorphous forms. It is found in coke in its amorphous form, while graphite and diamond provide examples of the crystalline form. Graphite, carbon black and coke breeze are all allotropes of carbon that conduct electricity, “breeze” being defined as small cinders or spherical particles which are formed as a by-product of the processing of coal or petroleum.
While such forms of carbon are sometimes used in shallow trench and deep well applications as backfill material without further processing, it is also known to modify carbon by means of various cementitious materials to improve its strength and structural integrity. The various types of materials which have been used to improve the properties of the carbon include hydraulic cements such as Portland cement, blast furnace slag, fly ash etc. Concrete and other cementitious compositions are normally prepared by mixing required amounts of hydraulic cement with fine and coarse aggregates and other additives known to the art, with required amounts of water. The terms ‘paste’, ‘mortar’, slurry and ‘concrete’ are used in the art: pastes are mixtures composed of an hydraulic cement binder, usually, but not exclusively Portland cement, which itself is a mixture of calcium, aluminum and ferrous silicates.
In the conductive concretes relevant to the present invention, the sand, stones and other minerals normally employed as aggregate are replaced by carbon in one of its forms. Optionally the various forms of carbon can be admixed with the aggregates and other additives commonly known in the art, provided that the concentration of carbonaceous material is sufficient to impart the necessary electrical conductivity. Such carbonaceous cements may be formed in place, or attached to the electrodes in the deep well cathodic process.
U.S. Pat. No. 6,121,543 (Hallmark) describes a groundbed electrode comprising a horizontally-oriented copper, or other electrically-conductive metal conductor, embedded in a cementitious sheath containing approximately equal parts of Portland cement and powdered crystalline carbon. The cementitious sheath may contain from approximately from 45 parts to 55 parts crystalline carbon powder, with the balance being Portland cement.
U.S. Pat. No. 3,941,918 (Nigol) discloses a conductive cement for use with electrical insulators in which graphite fibers are used to form a conducting network within a combination of Portland cement, graphite fibers and high structure carbon black to provide an electrically conductive cement with high compressive strength. Related applications of carbonaceous materials in a concrete matrix for use on various surfaces walkways, floors roadways and the like are described in U.S. Pat. Nos. 3,573,427 and 3,962,142, while U.S. Pat. No. 5,908,584 (Bennett) has described an electrically conductive building material comprising a mixture of graphite, amorphous carbon, sand, and a cement binder to shield building materials against electromagnetic radiation.
Deep Well Grounding Technique for Cathodic Protection
Deep well beds provide an effective method of increasing the life of subsurface metallic structures and the use of metallic anodes in combination with various carbon and graphite electrodes is now widespread. In this procedure the cost of electrode replacement is an important consideration, the rate of anodic consumption being dependent on the current density at the interface of the anode and soil medium. It has been found that a more uniform flow of current can be achieved if the anode is completely surrounded by a uniformly conductive carbonaceous backfill material.
According to the deep well technique a hole is drilled in the soil near the structure to be protected to an approximate depth of 150 to 450 feet, with a diameter of six or more inches. Typically the central anodes are composed of expensive materials such as mixed metal oxides or silicon-iron alloys, and under normal working conditions they may last from 5 to 10 years before suffering erosive failure. An anodic chain is then lowered into this hole and the hole is then filled with the backfill material, either in the dry form or optionally as an aqueous slurry.
It is well known to those skilled in the art that appropriate use of a suitable carbonaceous backfill material can double the lifetime of the system. The mechanism by which this is accomplished involves the sacrificial oxidation of the carbon in the backfill by the electrical current in preference to the central anode. The rate of loss of the carbon can be quantitatively estimated from known rate of oxidation during the electrolytic process.
Existing procedures for deep well electrical grounding, as described above, suffer from a number of shortcomings, one of the most serious being environmental contamination. An unavoidable by-product of the electrolytic reaction is the generation of gas, since, under the moist conditions commonly present in a deep well environment, the passage of one joule of electrical current results in the formation of one mole of gas. The most prevalent gas formed at the anode is oxygen, but if chlorides are present in the ground water, highly corrosive chlorine is also generated. If these gases are not able to escape from the system, cavitation and ultimately interruption of the circuit can result.
It is therefore important that the construction of a deep well anode be such that the anodic gases are able to escape. This is usually accomplished by utilizing a conductive backfill material to close the gap between the anode and the wall of the well. Utilization of backfill also allows the dimension of the hole to be large enough for easy emplacement of the central anode. There is however a serious problem associated with this common practice. Since coke breeze is very permeable to water, as well as to the anodic gases, its presence in deep well installations results in large quantities of water migrating between geological layers with unacceptable environmental consequences.
The high water permeability of coke breeze is of such concern that legislation in various States to ban the use of this material for this application is either pending or has already been enacted. It is therefore essential that any material used for backfill purposes have both acceptable impermeability to water and the requisite degree of electrical conductivity.
In recent years various disclosures have attempted to solve the problem associated with the vertical migration of ground water in deep well installations. One approach has involved covering the central metallic anode with some kind of sheathing material, or for example, by use of pre-packaged anodes emplaced in special containers or rigid cartridges (U.S. Pat. Nos. 3,725,699 and 4,400,259), or a more flexible construction which retains its shape and is thus more readily transported and installed. (U.S. Pat. No. 4,544,464 (Bianchi) uses a perforated disk filled with coke, to facilitate the flow of electric current between the central anode and the external casing, and backfill, composed of graphite and coke such that the anode is homogeneously surrounded by backfill in order to provide consistent current flow as the corrosion continues.
U.S. Pat. No. 4,786,388 (Tatum) discloses a low resistance non-permeable backfill for cathodic protection of subsurface metallic structures consisting of a mixture of carbonaceous materials, lubricants, Portland cement and water. In that process the slurry is pumped into an anode bed. To this end Tatum describes a method of pumping an electrically conductive cementitious backfill into the well in such a way as to produce a groundbed construction with a non-permeable concrete annulus in contact with the earthen bore, an improvement is said to avoid water quality degradation while at the same time achieving a low resistance ground contact.
U.S. Pat. No. 5,080,773 (Tatum) describes an electrical ground installed in the earth comprising an electrical conductor, a bore hole and a conductive non-porous carbonaceous cement composition surrounding said conductor and in contact with said rod by means of earth. These compositions are said to have enhanced conductivity, decreased porosity and a rate of set similar to that of conventional concrete.
“Watertightness” in the present context refers to the ability of a cementitious material such as concrete to hold back or retain water without visible leakage under given conditions of use or testing. “Permeability” refers to the amount of water migration through a material substance when the water is under pressure and is quantitatively determined according to ASTM D5084. The two properties are closely related, and both are conventionally measured in units of cm/sec, but will generally not be numerically identical because of differences between the prevailing conditions of use and the ASTM D5084 test conditions
When a mature good quality concrete is tested for permeability according to ASTM D5084 in which the length of sample is 4″ and the water pressure is 20 psi, the measured permeability is approximately 1×10−10 cm per second (reference: S. H. Kosmatka et al. 1995. Design and Control of Concrete Mixtures Ottawa: Canadian Portland Cement Association, pp. 8-9). For use in a cathode protection system, such a low degree of permeability does prevent the vertical transmission of ground water, but gases are also substantially prevented from escaping creating problems at cavitation, etc. referred to earlier. Such materials will hereinafter be referred to as “impermeable”. By contrast, with a permeability of about 0.05 cm/second, coke breeze backfill not only allows gases to escape but permits vertical migration of groundwater vertically through the system. A third example is bentonite which is regarded as a watertight product and exhibits a permeability of 10−7 cm/sec.
Although various conductive sheath compositions consisting of carbon in combination with cementitious binders have been directed to the problem of vertical water flow between aquifers, none has yet proved to be technically or economically suitable. One significant problem is the high cost of manufacture and installation. More importantly, none of these known systems provides a composition or method which simultaneously allows the gases to safely escape, while preventing the ground water from migrating. Porous carbonaceous backfill such as coke breeze is still widely used in deep well cathodic systems, however, problems arising from the vertical transmission of groundwater have become so serious that consideration is being given to banning the procedure.
In short, there has not been available to date any simple and inexpensive method for controlling the relative permeability of water and gases in deep well anodic installations.